1. Field of the Invention
This invention relates to digital-to-analog converters and, more particularly, to a digital-to-analog converter with an extended spurious free dynamic range (SFDR).
2. Description of Related Art
Many physical devices generate output signals which are analog or continuously varying. Today, signal processing is often accomplished using digital methods, and the processed signal is often converted back to analog form. Many types of converters exist which act as interfaces between analog devices and digital systems. These converters are used in a variety of applications, including testing, measurement, process control, and communications. Digital-to-analog converters (DAC) produce an analog output from a given digital input. The spurious free dynamic range (SFDR) for the DAC is often measured as the difference in decibels (dB) between the amplitude of the spurious distortion, such as intermodulation products, and the amplitude of the highest output signal. The SFDR performance of a DAC varies as a function of the amplitude of the digital signal to be converted. Usually, the greater the amplitude of the digital signal, the smaller the SFDR because as the amplitude of the digital signal increases, the amplitude of the analog output signal increases, but the amplitude of the resulting spurious distortion generally increases more than the amplitude of the output signal. Depending on the application, extending the SFDR of the DAC can enable the DAC to convert a greater range of digital signals to analog signals while maintaining or reducing the level of spurious distortion or to convert a given range of digital signals to analog signals while reducing the level of spurious distortion. For example, for discussion purposes, a 16 bit DAC could have a spurious free dynamic range of 90 dB measured by a high output level of 4 dBm and the spurious distortion level at -86 dBm. By increasing the relative difference between the amplitudes of the worst case spurious distortion and the high output signal, the SFDR of the DAC can be extended.